The invention relates generally to the field of digital color image processing and, more particularly, to a method for preserving image spatial detail and color content when adjusting the tone scale of a digital color image.
Existing methods for applying tone scale functions to digital color images include transforming the original digital color image into a luminance-chrominance representation and applying the tone scale function to the luminance channel while preserving the color difference signals. These methods of tone scale function application alter the sharpness and color appearance characteristics of the processed images.
In U.S. Pat. No. 5,012,333, Lee et al. described a method for applying a tone scale function to a digital color image with goal of preserving the spatial detail information. This method proposed separating the digital image into high spatial frequency and low spatial frequency components by using FIR filters. The tone scale function is then applied to only the low spatial frequency component of the original image. The final processed image is produced by adding the high spatial frequency component back to the tone scaled low spatial frequency component. Additionally, the method described in U.S. Pat. No. 5,012,333 involves transforming the original digital color image into a luminance-chrominance representation, applying the tone scale function to the luminance component with a spatial filtering technique, and preserving the chrominance components.
If the luminance contrast is modified and the chrominance contrast is preserved, the color saturation characteristics of the resulting processed image may appear artificial and unnatural. In an effort to address this problem, Viggiano and Wang proposed (1992 TAGA Proceedings: A Comparison of Algorithms for Mapping Color Between Media of Differing Luminance Ranges, pp. 959-974) a method of scaling the chroma signal of a digital color image by a factor midway between unity and the ratio of the L* range of the reproduction and the original. However, this solution does not anticipate tone scale functions that vary the level of compression dependent upon the intensity of the input. Thus, a single factor that scales the chroma signal of the input image is inappropriate for a wide range of possible tone scale functions.
In addition, in U.S. Pat. No. 5,446,504, Wada describes a method to correct color saturation when compressing the dynamic range of an image. However, this method once again determines the correction factor based upon the relationship of the input image dynamic range and the output image dynamic range.
In addition, in U.S. Pat. No. 5,638,138, Hickman describes a method of modifying the luminance signal of an input image, then xe2x80x9cmultiplying the color components by a transfer ratio of modified luminance to unmodified luminance to obtain modified color components.xe2x80x9d However, this method again does not consider that a tone scale function may consist of several ranges of varying level of contrast compression or enhancement.
The application of a tone scale function using spatial filtering techniques to a luminance signal representation of a digital color image described in U.S. Pat. No. 5,012,333 does preserve spatial detail in the processed image. However, while the spatial detail of the processed image is improved with this method, the color characteristics of the processed images can be affected. In addition, the size of the spatial filter used must be selected to achieve optimal results. Combining a spatial filtering method of applying a tone scale function to the luminance signal and the chrominance signal modification is natural. The processed images with the combination of these methods produces images with both improved spatial detail and more natural color characteristics.
The method of applying a tone scale function to a digital color image with spatial filtering techniques affects the color characteristics. In particular, the size of the spatial filter used in the tone scale function application affects the color characteristics. None of the methods described in the prior art involves the modification of the image chrominance characteristics based upon the method of spatial processing used to apply a tone scale function to the luminance signal.
There exists an inverse relationship between the size of the spatial filter used to apply a tone scale function to the luminance signal and the degree to which the chrominance signals should be modified. Thus a need exists for a method that coordinates the modification of the chrominance signals and the luminance signal while applying a tone scale function to a digital color image.
The object of the present invention is directed to providing an improved method for preserving spatial detail and color content when adjusting the tone scale of a digital color image. Briefly summarized, according to one aspect of the present invention, the invention resides in the steps of: (a) receiving a digital color image; (b) providing a tone scale function for adjusting the tone scale of the digital color image; (c) providing a spatial filter for adjusting the tone scale of the digital color image; (d) transforming the digital color image into a luminance signal and chrominance signals; (e) using the tone scale function, the spatial filter, and the luminance signal to produce an enhanced luminance signal; and (f) using the tone scale function, the spatial filter, the chrominance signals, and the chrominance signals to produce enhanced chrominance signals.
The present invention has the advantage of preserving spatial detail and color content while altering the tone scale of a digital color image. A key element of the present invention is the coordination of the chrominance modification and the luminance modification based on the characteristics of a digital filter.